This invention relates to a power source for gas discharge lamps and other loads operated at high frequency, and in particular is directed to a new and improved power source suitable for energizing fluorescent lamps with improved efficiency and reduced heating.
The basic design for a power source utilizing a swinging choke inverter is shown in U.S. patent to Campbell U.S. Pat. No. 3,396,307. Variations on this basic circuit are shown in U.S. patent to Pierce U.S. Pat. No. 3,889,153 and copending applications of Perper Ser. No. 596,069 filed July 15, 1975, now U.S. Pat. No. 4,005,335, and Ser. No. 611,897 filed Sept. 10, 1975, now U.S. Pat. No. 4,017,785, and assigned to the same assignee as the present application. It should be noted that the circuitry of Campbell, Pierce, and Perper is primarily representative of voltage feedback in that a voltage from the output transformer is derived in a secondary winding, processed, and supplied to the transistor base. Similarly, the circuitry of U.S. Pat. Nos. 3,501,674 (Moore) and 3,882,354 (May) is primarily representative of voltage feedback, since a voltage from the output transformer is also used.
In a power oscillator, the quantity fed around is likely to contain components representative of both voltage and current, so that the concepts of voltage and current feedback are inexact. Nevertheless if the feedback is primarily of one type or the other, the nomenclature is reasonable and essentially valid.
In the circuitry hereinafter disclosed, use is made of the fact that the oscillator current comprises a steeper and more suitable switching signal than collector voltage. The feedback voltage is primarily representative of such current. To qualify as such, the feedback winding must produce a voltage drop representative of the current flowing through a small resistance in series with the transistor or the load. A separate transformer for feedback permits current feedback; the use of a secondary from the transistor output transformer, as in Moore or May, inherently produces voltage feedback, and distinguishes from current feedback as herein disclosed.
The distinction between voltage and current feedback as herein used, is apparent, as seen on an oscilloscope, in the wave shapes produced. In both cases, the primary of the output transformer may be considered to function as a swinging choke. The swinging choke inverter uses a power transistor operated as a switch for switching current through the primary of a transformer. However the transistor is not a perfect switch and there are power losses in the switching operation resulting in undesired heating and reduction in efficiency. There is current flow during the time of voltage turn on and turn off, and the energy dissipated produces heat. Also, an appreciable voltage-current product exists during the conduction interval (the on time) of the transistor due to the saturation voltage not being minimized.
Accordingly, it is an object of the present invention to provide a new and improved power source which reduces the energy losses during the turn on, turn off and conduction intervals, with a resultant reduction in heat loss and improvement in efficiency. It is a specific object of the invention to provide an improved circuit utilizing current feedback in place of the conventional voltage feedback and to increase the base current during the conduction interval while shortening the turn on and turn off times. An additional object is to provide such a power source incorporating voltage limiting and current limiting features, and one which may utilize more than one power transistor for increased load handling.